Apparatus for growing crystals



fx q@ Q31 Am Aug. 17, 1954 N. N. ESTES APPARATUS FOR GROWING CRYSTALS 7 Sheets-Sheet l Filed March 21, 1949 N /V Estes @www Aug. 17, 1954 N. N. EsrEs APPARATUS FOR GROWING CRYSTALS 7 Sheets-Sheet 2 Filed March 21. 1949 Aug. 17, 1954 N. N. ESTES APPARATUS FOR GROWING CRYSTALS '7 Sheets-Sheet 3 Filed March 21, 1949 Jn/verdon NN Eses M @Ea/WM 3N. N.

Aug. 17, 1954 N. N. ESTs-:s

APPARATUS FOR GROWING CRYSTALS 7 Sheets-Sheet 4 Filed March 21, 1949 Aug. 17, 1954 N. N. ESTES 2,686,712

APPARATUS FOR' GROWING CRYSTALS Aug. 17, 1954 N. N. EsrEs 2,586,712

APPARATUS FOR GROWING CRYSTALS Filed March 2l, 1949 7 Sheets-Sheet 6 Aug. 17, 1954 N. N. ESTES 2,586,712

APPARATUS FOR GROWING CRYSTALS Filed March 2l, 1949 7 Sheets-Sheet '7 Steps of Arm |50 Between Discharges` of Chamber |79 l 2 3 4 5 6 7 8 9 l0 Successive Periods of Operation of Apparatus Time NN Estes Patented Aug. 17, 1954 UNITED STATES PATENT OFFICE (Granted under Title 35, U. S. Code (1952), s sec. 266) 3 Claims.

This invention relates to methods and apparatus for the production of clear and flawless crystals from materials that have a nearly flat or negative slope solubility curve such, for example, as lithium sulphate monohydrate. More specifically, the invention includes a method for growing clear and awless crystals which involves steps for controlling the formation of spurious crystals due to evaporation at the surface of the solution comprising agitating the surface of the growing solution and condensing water vapor on areas of the container or crystal support above the growing solution to automatically dissolve crystals deposited thereon before growth starts and thereby prevent crystals from forming thereon. The invention also pertains to methods in which the size and flawless quality of the crystals are controlled by regulating the hydrogen ion concentration, or pH value and the growing temperature of the crystallizing solution. The quality of the crystals with respect to flaws is further controlled by the method of cutting the crystal seeds from a crystal bar. The invention also includes apparatus for automatically maintaining the temperature, pI-I value and the supersaturation of the solution within a narrow range during a predetermined program of crystal growth.

Methods of growing crystals from solutions with a positive slope solubility curve which have been heretofore proposed include methods whereby the crystallization is carried out either by lowering the temperature of the crystallizing solution without evaporation or by evaporation of the solution at constant temperature or a combination of both. The aforementioned methods apply only to those crystal materials that have a positive slope solubility curve, such, for example, as Rochelle salt crystals. These methods have not been satisfactory for growing optically clear crystals from materials that have a flat or negative slope solubility curve such as lithium sulphate monohydrate for the reason that it has not been possible to obtain a sufficient rate of growth to be practical for growing these crystals free of internal strain. The novel methods and apparatus of the present invention for growing flawless crystals overcome to a large extent, the disadvantages of the prior art methods and apparatus. To grow clear and flawless crystals of lithium sulphate monohydrate by the methods of the present invention it is necessary to use seeds of this materials that have faces parallel to the major natural faces of the crystal. Further, clear and flawless crystals of Aif.)

lithium sulphate monohydrate may be grown only when the hydrogen ion concentration or pH value and the temperature of the solution are maintained within narrow limits. Under these conditions clear and awless growth occurs in length and width but growth does not occur in thickness. To obtain thicker seeds it is necessary to grow crystals in a solution having a pH value for the hydrogen ion concentration below the optimum. This causes strains to be developed in the crystals and results in an increase in thickness. The strains developed thereby also cause iiaws and imperfections in the crystals. The flaws and imperfections which occur as a result of seed planting and which inherently produce rounded corners with initial immersion in the solution are grown out by cutting seeds from the clearest sections and regrowing at the crystal corners. This results in flawless crystal seed growths of increased thickness.

Apparatus for growing crystals which have been produced heretofore provide a constant supersaturation of a crystallizing solution with a positive slope solubility curve by evaporation of the solution at a constant temperature or by lowering the temperature without evaporation. The process of lowering the temperature without evaporation for a solution of lithium sulphate monohydrate has the disadvantage of requiring such a large amount of original solution, that program control means, based on temperature changey for maintaining a constant supersaturation would be impractical. The present invention, therefore, utilizes apparatus which controls the supersaturation of the crystallizing solution by evaporation of the solution at constant temperature.

In accordance with the preferred embodiment of the invention a tank is provided having a cover on which is supported means comprising a vertical shaft with a plurality of arms for supporting the crystals grown thereon. Inside of said tank is mounted a condenser connected in siphon relation to dispensing means outside of said tank. Dispensing means are also provided outside of said tank for transferring make up solution to the tank during the crystallizing process. An automatic program control system is connected to operate both of the dispensing means described herein in accordance with a predetermined schedule of operation.

An alternative embodiment of the invention involves a blower system by which moisture laden air is conveyed from the tank to an external condenser, the period of operation of the blower being controlled by the aforedescribed program control system.

One of the objects of this invention is to provide a, method and apparatus for growing clear and awless crystals of a material having a flat or negative or slightly positive slope solubility curve.

Another object of the invention is to provide a method for growing clear and flawless crystals from a solution of a. material that has a nearly flat or negative or slightly positive slope solubility curve and having provision for stirring the solution to develop frothless waves thereby to reduce the possibility of spontaneous seeding due to increased saturation resulting from evaporation at the surface of the solution.

A further object of the invention is to provide a method for growing clear and iawless crystals from an enclosed solution of a material that has a nearly at or negative or slightly positive slope solubility curve and having provision for condensing water vapor on the portion of the enclosure above the surface of the solution thereby to reduce the saturation of solution washed up on this surface and thereby reduce the possibility of spontaneous seeding thereon.

Still another object of the invention is to provide clear and flawless crystals of predetermined dimensions from a material having a flat or negative or slightly positive slope solubility curve.

Another object resides in the provision of a method and apparatus for growing clear and awless crystals from materials having the aforedescribed solubility characteristics under controlled conditions of hydrogen ion concentration.

Another object is to grow crystals of the aforedescribed characteristics in which provision is made for first growing a crystal along all axes and thereafter growing a crystal from a seed cut from a clear and awless portion of the first crystal along certain axes with growth along .at least one axis inhibited, thereby eliminating incipient aws introduced during the rst growth.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a diagrammatic view of a crystal growing system according to the preferred embodiment of the invention and employing a condenser positioned inside of the growing chamber;

Fig. 2 is a diagrammatic view of an alternative embodiment of the invention illustrating a condenser positioned outside of the growing chamber and a blower for circulating the air;

Fig. 3 is a top view of the device that dispenses water from the condenser in the growing chamber of Fig. 1;

Figs. 4 and 5 are views in :action through the water dispensing device of Fig. 3 taken along the line 4 4;

Fig. 6 is a view of the condenser that is positioned inside the growing chamber of Fig. l;

Figs. "l thru 11 are views in section through the condenser of Fig. 6;

Fig. 12 is a view illustrating the connection between the condenser of Fig. 6 and the dispenser of Fig. 3;

Fig. 13 is a view in section of the condenser positioned outside of the growing chamber in the 4embodiment of the invention illustrated by Fig. 2;

Referring now to the accompanying drawings in which like numerals of reference are employed to designate like parts, and more particularly to Fig. 1 thereof in which reference character 4 designates a jar containing the crystallizing solution. Jar 4 is located within a tank provided with a cover 1. Within tank I and around jar 4 is a suitable bath 3 such as water that supplies heat to the solution in jar 4. Calrod Heater 2 supplies the heat lost by radiation, conduction and convection so that the temperature within container 4 may be maintained within narrow limits. The current supplied to Calrod Heater 2 is controlled by the apparatus illustrated by 26, Fig. 14. Reference character 24 designates a well known type of thermal switch with taps A and B. Connected to tap B and the volt A. C. source |69 is the armature of the relay indicated generally by 25. The armature 21 and contact 28 of relay 25 are arranged to connect the winding of relay 29 to A. C. source |69. Armature 21 is biased against contact 2B by spring 34. The armature 3| and contact 32 at relay 29 are arranged to connect the Calrod Heater 2 to the 220 volt A. C. source 33. Armature 3| is biased to open position by spring 35.

When the temperature of the bath 3 drops below the desired value, the mercury in thermal switch 24 drops below point B and thereby opens the circuit energizing solenoid 25. Armature 21 is then pulled against contact 28 by spring 34 thereby energizing solenoid 29. Energizing solenoid 29 causes the armature 3| to be pulled against contact 32 which closes the circuit that applied the energy from the A. C. source 33 to Calrod Heater 2. When the bath 3 is heated to the desired temperature, the mercury of thermal switch 24 will rise until contact is made at B. When contact is made at B, coil 25 is energized and armature 21 is pulled away from contact 28, coil 29 is deenergized and spring 35 pulls armature 3| away from 32 thereby opening the circuit energizing heater 2.

Mounted on shaft 8 that passes through the lid 5 of container 4 is a tree 6. Crystal seeds are mounted on the extremities of the three lower pairs of arms of the tree. Blocks are mounted on the extremities of the upper pair of arms in order that suicient stirring of the top layer of solution will be accomplished to reduce the possibility of spontaneous seeding due to evaporation at the surface of the solution. The tree 6.

and the blocks mounted thereon, are rotated in such a manner as to produce frothless waves approximately 11/2" high.

Whereas the specification has herein described the crystal supporting tree with blocks mounted thereon as specific means for producing frothless waves, it will be understood that other means such as separate drive for the stirring blocks could be used to produce the same result. Further, it will be understood that blocks of various shapes positioned at various depths below the surface of the crystallizing solution will produce frothless waves of the type desired for growing clear and flawless crystals.

'Ihe upper area of the container 4, i. e. the

area above lid 1 is maintained at a temperature below the temperature of the solution so that water will condense on this area and dissolve spurious crystals that may form on this area due to splashing thereon during the start up period. Further, the condensed liquid reduces the saturation of solution washed up on the area above lid 1 during the growing process and thereby reduces the possibility of spontaneous seeding on this area.

One of the factors influencing the rate of growth of crystals is the uniform application of the crystallizing solution to the growing faces of the seeds. This is accomplished by careful control of the circulation around the growing seeds. Proper circulation of the solution around the growing seed is obtained, in the instant invention, by periodically reversing the direction of rotation of the tree to which the crystals are attached. It should be understood however that satisfactory crystals may be grown at a reduced rate of growth by using a method which does not involve reversing the direction of rotation of the tree to which the crystals are attached. The reversing of tree -6 is controlled by motor reverse control 5|. Keyed tp ft 8 of isAa pulley rgtaitedrby/a belt I I driven by reversible motor I0. W

' Whereas the specication has herein described the crystal tree as being supported by a shaft that extends through the lid on the top of the container it will be understood that the tree could be driven by a shaft extending through a suitable packing gland in the bottom of the container.

Crystal growing with the apparatus of the subject invention is done by evaporation of the solution at constant temperature.

The withdrawing of salt from the crystallizing solution and the depositing of it on the crystal seed makes it necessary to remove water from the solution. This is necessary in order to maintain the saturation of the crystallizing solution at the level of supersaturation necessary for the growing of crystals. Removal of water from the solution is accomplished by condenser |2 shown by Figs. 6 thru l1. The cooling element I3 of condenser I2 has a U-shaped passageway 54 which is connected to a cold water supply tube I4. Tube I5 drains the cooling water from the condenser. Fins I6 are attached to cooling element I3 to increase the heat transfer surface of the condenser. Container I1 is provided with ports I8 to permit the water vapor to circulate around the cooling surface of the condenser. The water that forms on the cooling surface is collected in the bottom of container I1. When the water reaches the top of lip I9 it closes ports I8 and stops the circulation of water vapor around the cooling ns which greatly reduces the rate of condensation. This assists in preventing the formation of a more saturated layer on the surface of the solution which would increase the probability of spontaneous seeding.

Inserted in one end of passageway 20 of element I3 is a beveled tube 2 I. Attached to the other end of passageway 20 is a tube 22 that is connected to a short tube |80 inserted in Lucite block I8I of device |15. Passageway |82 inblock I8| connects the chamber |19 to tube |80. Capillary tube |88 is mounted in the top of measuring chamber |19 to provide an air vent for this chamber. The device is mounted in relation to the container I1 of condenser I2 so that water will sphon from container I1 into chamber |19. The relative position of the chamber |19 and container I1 is such that chamber |19 will contain a predetermined volume when the liquid level in chamber |19 equals the liquid level in container I1 at which time the siphoning stops. The aforesaid predetermined volume being determined by the size of seed crystals and the rate of growth desired.

The siphoning of liquid from container I1 to chamber |19 is initially started by means of external suction and is caused during the growing cycle by the discharging of liquid from chamber |19. The liquid discharged from chamber |19 is conveyed to beaker 52. Chamber |19 is discharged by energizing solenoid coil |14 which pulls core |83 and plug valve |18 to the right. The valve |18 is only opened for a brief period of time since the coil |14 is only energized momentarily.

Passageway |86 from chamber |19 is of sumcient size however, to cause complete discharge of the liquid in chamber |19 during the brief interval of time that plug valve |18 is drawn to open position. The passageway |82 is also sufriciently small so that a neglible amount of liquid enters chamber I 19 during the brief period of time that plug valve |18 is in the open position. The core |83 is biased toward the left by spring |84. When the coil |14 is de-energized therefore, plug valve |18 is pushed to the left until it is in abutting relation with valve seat |85. Valve |18 is equipped with a washer to permit a liquidtight seal between the valve and seat. When the plug valve |18 has been seated chamber |19 starts to ll as a result of the siphoning from container I1 through tube 2 I, passageway 20, tubes 22 and |80, and passageway |82.

Although a siphon arrangement has been described herein for transferring liquid from condenser I2 to dispensing device |15 it should be understood that other arrangements which do not involve a siphon may be used to obtain the same result.

As the crystals grow the surface areas presented to the crystallizing solution are gradually increased and larger quantities of salt are deposited on the crystals which necessitates the removal of water from the solution at an increasing rate to maintain a constant super saturation of the crystallizing solution. Program control |01 to be described subsequently is constructed and arranged to cause the rate at which device |15 is operated for discharging liquid to increase directly in accordance with the rate of development of the surface area of the crystals.

During the crystal growing process it is necessary to add make-up solution to container 4 in order that the level of liquid in this container will be maintained at a constant value. The make-up solution in container 53 is conveyed to container 4 through tube 39, solenoid valve 38 and hand valve 31. The solenoid valve 39 is opened by the same current impulse from program control |01 that energizes solenoid |14 of device |15. Although in the instant invention make-up solution is conveyed to the growing chamber simultaneously with removal of condensate from this chamber it will be understood that the make-up solution need not be added simultaneously with removal of condensate. Make-up solution should be added as often as is necessary to maintain the stirring blocks submerged in the growing solution.

The saturation and pH value of the make up solution are both preferably controlled and may be varied during the growing cycle to maintain an optimum supersaturation and prevent the pH value of the growing solution from exceeding the maximum value of the desired range.

The valve 38 is not closed simultaneously with the deenergizing of solenoid 14. It is necessary for the solenoid valve 38 to remain in the open position a predetermined period of time so that the proper amount of liquid will be added slowly to container 4. Time delay 36 of a well known and conventional type is used to delay the de-energizing of solenoid 39 the required period of time. Time delay 36 is energized by source |69 through leads |90 and |9|.

Referring to Fig. 2, there is shown thereon an alternative arrangement for growing crystals by evaporation of the solution at constant temperature in which dehydrated air is passed through the space above the crystallizing solution for removing the excess water Vapor in container 4. Dehydration of the air is accomplished by condenser 40. Blower 4I draws the dehydrated air from condenser 40 through duct 42. The dehydrated air is then forced through valve 43, duct 44, meter 45, duct 46, and diffuser 41 to container 4. After the air becomes laden with moisture in container 4 it is drawn through duct 48 to condenser 40 where the moisture is removed. The cooling water enters condenser 40 through tube I4, circulates around the corrugated cooling surface 50 and discharges through tube I5. The walcer condensed in condenser 40 is discharged through tube 5| to beaker 52.

It is app-arent from Fig. 2 that blower 4| and solenoid 38 are connected in parallel across time delay 36. Blower 4| operates therefore at the same time that solenoid valve 38 is energized. Moisture is therefore removed from container 4 simultaneously with the addition of make-up fluid from container 36. Program control |01 is constructed and arranged to intermittently operate blower 4| and solenoid 38 in accordance with a predetermined schedule of crystal growth.

Time clock 60 of program control |01 is arranged to close switch I| momentarily after the lapse of a constant and predetermined period of time such as every l2 hours or every day. The closing of switch |0| causes the discharge of condenser |02 through the stepper switch coil 6|. Condenser |02 is charged from selenium rectier |03 through leads |04 and |05 and resistance |06 after switch IDI has been opened. When condenser |02 discharges through coil 6I the armature (not shown) of the switch is pulled back. When the condenser has become discharged a predetermined amount causing sumcient reduction in the energization of coil 6| to release the armature, arms 62 to 61 of stepper switches, indicated generally by reference characters 68 to 13 respectively, are advanced by the armature one step in a clockwise direction. Arms 63 and 66 of switches 69 and 12 respectively are connected to thQarms 64 and 61 of switches 10 and 13 respectivjly by lead 81. The contacts of switches 10 andw13 are connected to the condenser 90 by lead 89. Condenser 90 is grounded at 9 I.

Connected across the contacts of stepper switches 69 and 12 are resistors of equal resistance. The resistors connected across the contacts of switch 69 are grouped in three banks of resistance 14, 15, and 16 and the resistors across the contacts of switch 12 are grouped in one bank of resistance 11. The four banks of resistance 14 to 11 are connected to the cathode 19 of full wave rectifier tube 18 by leads 80, 8|, variable resistance 82, leads 83 to 85, and resistance 86.

The plates of rectifier tube 18 are connected to the transformer secondary 93. The cathode 19 of tube 18 is heated by energy derived from secondary coil |68. Secondary coils 93 and |68 are energized by primary coil |99 which is connected to alternating source of energy |69 through fuze |10. The center tap 92 of transformer secondary 93 is grounded at 94. Condenser 90 is charged by full wave rectifier tube 18 through a circuit including the following: cathode 19 of rectifier 18, resistance 86, leads 85, 84, 83, variable resistance 82, leads 8| and 80, one of the resistances R1, R2, R3, and R4, one of the four banks of resistance 14 to 11, arm 6-3 or 66. for example, bank 14 and arm 63, as indicated, lead 81, arm 04 or 61 of switch 10 or 13 respectively, this being arm 64 as indicated, and lead 89 to condenser 90. Condenser 90 is connected to resistance 95 by lead 89, switch 10 or 13, and leads 81 and 88. Resistance 95 is connected to the grid 91 of thyratron tube 96. Thyratron tube 96 and an RC circuit including the resistance of one of the banks of resistance 14 to 11 and condenser 90 comprise a relaxation oscillator. The frequency of the oscillator depends upon the time required to charge condenser 90. The rate of charging of condenser 90 depends on the amount of resistance of resistance banks 14 to 11 and also on the resistance of R1, R2, Re or R4 inserted in the circuit between rectier tube 18 and condenser 90. The amount of resistance of the particular bank of resistance in series with condenser 90 at a particular time will be progressively decreased as the arms 63 and 66 are stepped in a clockwise direction under the control of time clock 60. The time interva1s are increased substantially between periods of operation I0 and II, when R2 is inserted in the circuit with resistance in bank 15, since R2 is substantially greater than R1. Likewise, when arm 63 contacts bank 16, R3 which has a substantially higher resistance than R2 is placed in the circuit with condenser 90, and the time constant is therefore, substantially increased between operation periods I4 and I5. Another similar increase in the time constant also occurs between periods of operation 23 and 24 when R4, of larger magnitude than R3, is placed in the circuit.

Thyratron tube 96 fires when the charge on condenser 90 is adequate to render the tube conductive. The ring of tube 96 completes a circuit which energizes coil I|5. This circuit includes cathode 19, resistance 86, leads and |96, coil H5, leads ||4 and ||3, plate 99, cathode 98, leads ||2, III, |36, and I|0 and center tap 92.

The energizing of coil I5 causes armature I6 to move into engagement with contact I I1. which closes a circuit from condenser to ground 94 thereby completely discharging condenser 90. This circuit includes lead 89, switch 10 or 13, switch arm 64 or 61, leads 88 and I|8, armature ||6, leads ||9. III and |09.

Energizing coil ||5 also causes armature |20 to move into engagement with contact |2| which closes .a circuit from selenium disc rectifier |03 for energizing coil |22 comprising leads |04, |08 and |23, armature |20, contact |2|, resistance |24, coil |22, and lead back to rectier |03. Selenium disc rectier |03 is energized by secondary coils I1| and |12, which are energized by primary coils |99 and |13.

The energizing of coil |22 causes armature |26 to move into engagement with contact |28. The

amature |26 and contact |28 close a circuit for charging condensers |3| and |32 from rectifier |03, through leads |04, |08, and |46, resistance |30, lead |29, armature |26, contact |28, and resistances |33 and |34 to condenser |3| and resistances |33 and |35 to condenser |32. When condensers 3| and |32 are charged adequately, thyratron tube |31 will become conductive and will fire. The firing oftube |31 causes a voltage drop across resistance |38 through a circuit comprising cathode 19 of rectifier 18, resistance 86, leads 85 and 84, resistance |36, plate |39, and cathode |40 of thyratron |31, leads |4|, I||, |36 and to center tap 92. The voltage drop across resistance |38 is of sufficient magnitude to reduce the voltage applied to plate |39 below the value required to render the thyratron tube |31 conductive. Thyratron tube |31 is therefore extinguished. Since the plates 99 and |39 of tubes 96 and |31 respectively are connected by leads ||3, condenser |42 and lead |43, the voltage on plate 99 will fall to the value of the voltage on plate |39. The reduction of voltage applied to plate 99 causes tube 96 to be extinguished. Thus the tube |31 snuffs out tube 96.

The energizing of coil |22 causes armature |25 to move into engagement with contact |21 simultaneously with the engagement of armature |26 and contact |28. The engagement of |25 and |21 closes a circuit from rectifier |03 to stepper coil |44 of stepper switch |49. This circuit includes rectifier |03, leads |04, |08 and |46, resistance |30, armature |25, contact |21, lead |41, stepper coil |44, and leads |48, |20, and back to rectier |03. Stepper switch |49 comprises stepper coil |44, reset coil |45, rotating arm |50 and a plurality of contacts on which the rotating arm impinges. The stepper coil 44 causes the rotating arm |50 to move to the next succeeding contact whenever the coil receives an energizing impulse.

Connected to contacts C2, C4, C6, and C| 0 of stepper switch |49 are contact banks |56, |51, |58 and |59 respectively of switches 68 and 1|. The arms 62 and 65 of swithces 68 and 1| respectively are connected together by lead |60.

Coil |6| is energized by rectier |03 through a circuit including the following: leads |95. coil |6|, leads |92 and |60: arm 62 and one of the banks of contacts |51, |58 and |59 or arm 65 and contact bank |56; the appropriate lead to the contacts on stepper switch |49, arm 50 and leads |94, |46. |08 and |04 to rectifier |03.

Energizing coil |6| causes armature |62 to engage contact |63 which closes a circuit between rectifier |03 and reset coil |45 comprising the following: lead |04, resistance |93, armature |62, Contact |63, lead |66, reset coil |45, and leads |48, |09 and back to rectifier |03. The energizing of reset coil |45 resets arm |50 at position X from which it again steps forward in a clockwise direction in accordance with the occillation of tube 96.

The energizing of coil |6| also causes armature |64 to engage contact I 65 which completes a circuit from the source of alternating current |69 comprising fuze |10, lead |11, coil |14 (Fig. 4), armature |64. contact |65 and lead |89. The energizing of coil |14 causes the withdrawing of valve plug |18 and discharging of liquid from chamber |19. Leads |11 and |89 are also connected to time delay 36, as shown in Fig. 1, which in turn actuates solenoid valve 38. In the modication of Fig. 2, leads |11 and |89 are con- 10 nected to time delay 36 which in turn actuates both blower 4| and solenoid valve 38.

From the foregoing it is apparent that the firing of thyratron tube 96 causes the energization of coils I|5, |22 and stepper coil |44 in the order named. The stepper coil |44 advances the rotatable arm 50 one step into engagement with the next succeeding contact each time thyratron tube 96 fires. The rate of firing of thyratron 96 and therefore the rate at which the rotating arm |50 steps around the contacts depends upon the rate of charging of condenser 90. The rate of charging of condenser 00 depends on the amount of resistance of resistance bank 14, 15, 16 or 11, as the case may be, that is connected in series between rectifier tube 18 and condenser 90. 'Iime clock 60 in conjunction with relay coil 6| and switches 68, 69, 1| and 12 controls this resistance. Time clock 60 in conjunction with switches 68 and 1| also controls the number of steps of stepper switch |49 before resetting of the switch. When arm 62 of switch 68 is resting on any contact of contact bank |59 the arm 50 advances 10 steps to contact C|0 before resetting. However, it should be noted that the rate of stepping of arm |50 increases as the arm 62 is advanced in a clockwise direction along contact bank |59 due to the progressive reduction of the quantity of resistance 14 connected in the aforedescribed RC circuit. When arm 62 is contacting contact bank |58 or |51 or when arm 65 is contacting bank |56 the arm |50 of stepper switch |49 advances 6, 4 and 2 steps respectively before resetting.

By careful analysis of Fig. 16 it is clearly apparent that between periods of operation I0 and the time interval between steps of arm |50 suddenly increases, but, since the number of steps required between discharges of chamber |19 simultaneously decreases from 10 to 6, the time between discharges decreases slightly. A similar result is obtained between periods of operation |4 and I5 and between 23 and 24.

It is noted that the energizing of coil 6| causes the simultaneous resetting of contact arm |50 and discharging of measuring chamber |19. It is evident, therefore, that the rate of discharging of chamber |19 is a function of the particular contact bank which arms 63 and 66 may impinge on as well as the position of one of these arms on a contact bank. The particular contact bank determines the steps arm |50 makes before resetting; the position of the contact arm on the bank of contacts controls the rate of stepping of arm |50.

The performance of the timing apparatus is best illustrated by the chart of Fig. 16 in which the ordinate represents the successive periods of operation of the crystal growing apparatus and the upper abscissa represents the steps made by the arm |50 between each discharge of chamber |19. The lower abscissa represents time and the slanting line on the right of the chart represents the time between discharges of chamber |19 for each period of operation. It is noted that the time between discharges of chamber |19 is gradually reduced from the first period of operation until the iinal period of operation. This is accomplished by varying the number of steps made by arm |50 between discharges and the time between steps.

Although electrical timing means has been set forth hereinbefore it is to be understood that any variation in the timing means resulting in the production of preselected control steps may be utilized for this invention.

In order to grow clear crystals which are free of internal strain with the hereinbefore described novel apparatus it was also necessary to develop new methods of utilizing this or similar apparatus for growing clear and flawless seeds and methods of growing crystals therefrom of the desired type and size.

To grow original seeds of lithium sulphate monohydrate which involves growth in all directions, a Supersaturated solution with a pH range of 3 to 4 and a temperature of approximately 100 C. should be used. The pH range of 3 to 4 is below the optimum for the growing temperature of 100 C. This low pH may be considered as a means of producing strains in the growing seeds and thus results in an increase in thickness. To start crystal growth, a few crystals of lithium sulphate monohydrate salt are added to the crystallizing solution while the tree 6 is revolving therein. A few of these salt crystals will then attach to the arms of the tree and crystals of the crystallizing solution will form thereon. The resulting crystals will contain cracks and flaws due to the strains that produce growth in thickness. Seeds should be cut from the clearest section of the crystals and regrown in a solution with a pH of 6.5-7.3 at approximately 100 C. which will cause clear and flawless growth in length and width but will not cause appreciable growth in thickness. By this procedure the cracks that would result from growth in thickness will be eliminated. Seeds may be cut from the clearest portion of the resulting crystal and again regrown for further elimination of flaws.

The regrowing of seeds in the 6.5-7.3 pH solution should be repeated until clear and flawless crystal bars have been grown which are suitable for seed material. Seeds may be cut from these bars and other crystal bars may be grown therefrom. Seeds should be cut from the bars in such a manner that the seed faces will be within 11/2" of the major natural faces of the crystal. Crystals grown from seeds which do not have faces substantially parallel to the major natural crystal faces will contain a multitude of cracks and flaws.

Clear and flawless crystal bars suitable for piezoelectric applications may be grown from seeds grown by the hereinbefore described process in suitable solutions of lithium sulphate monohydrate.

Supersaturated solutions of lithium sulphate monohydrate of a pH in the range between 6.5 and 8.1 and of a temperature in the range between 70 and 105 C. will support the growth in length and width of clear and flawless crystal bars of a thickness determined by the thickness of the seed material. It should be noted that these optimum ranges of the pH va-lue for the hydrogen ion concentration vary with impurities of the salt and with the tendency of the solution to cause spontaneous seeding. In the lower part of the temperature range the pH value is more critical and should be somewhat higher for optimum growing of the crystal.

Crystal bars greater in thickness than the thickness of available seeds may be obtained by growing them from the seeds in a solution with a pH range of 4 to 5 at a temperature of approximately 100 C. The pH range of 4 to 5 is below the optimum for 100 C. growing temperature which, as hereinbefore explained, may be considered as producing strains in the growing bars which results in an increase in thickness of the crystal bar. The thicker crystal bars will contain cracks and aws due to the strains that produce growth in thickness.

The strains and cracks may be eliminated by obtaining seeds from the clearest area of the thicker crystal bars and growing these seeds in a 6.5-7.3 pH solution at approximately C., which as hereinbefore explained, does not cause appreciable growth in thickness but does produce clear growth in length and width. The cycle described herein for eliminating flaws and for increasing thickness may be repeated until clear andflawless crystal bars of the desired thickness have been produced.

The temperatures specified herein for use with various values of pH for the hydrogen ion concentration are optimum values for obtaining the maximum rate of crystal growth. It should be understood that satisfactory crystals may be grown at a rate of growth less than the maximum by growing these crystals at a temperature slightly more or less than the optimum.

The values of pH noted above were indicated by an electrode potential method of measurement using a Leeds and Northrup electrometer. and corresponding values as measured by organic indicators which change in color are somewhat higher for the same solution, particularly when measurements are made in the upper range of pH values.

Whereas the specification has heretofore set forth certain examples of a method and apparatus suitable for growing lithium sulphate monohydrate crystals under controlled conditions it should be understood that the same or similar apparatus and methods can be used to grow crystals from any solution of a material having a flat, or negative, or only slightly positive slope solubility curve.

Obviously many modications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. Apparatus for growing crystals comprising a closed container adapted to be partially filled with a growing solution of the material to be crystallized, means for supporting a seed crystal in said growing solution, means for controlling the temperature of said growing solution, a condenser mounted on said container and having a condensing unit with a collecting chamber positioned in the vapor space within said container above the liquid level of said growing solution and below the upper enclosing portion of said container, means for removing condensate from said collecting chamber, solenoid means for operating said last named means, means for introducing make up solution to said container including a valve actuated by said solenoid, control means for actuating said solencids at predetermined intervals, said control means including means for progressively decreasing said time intervals, and time delay means operatively connected between said control means and said solenoid valve for delaying the closing of said valve.

2. Apparatus for growing crystals comprising a closed container adapted to be partially lled with a supersaturated growing solution, means for supporting a seed crystal in said growing solution, means for causing relative motion between said support and said growing solution, means for controlling the temperature of said growing solution, a condenser mounted on said container and comprising a cup-shaped member arranged upright within said container, said cupshaped member having a cooling element therein, a plurality of openings extending laterally through said cup-shaped member and adapted to facilitate access of vapor to said cooling element until condensed vapor has f1l1ed said member to a predetermined height, and means for removing condensate from said member.

3. Apparatus for growing crystals comprising a closed container adapted to be partially lled with a. solution, a device disposed within said container above the surface of said solution for condensing vapor evaporated therefrom, said device comprising a cup-shaped member and an elongated cylindrical member for depending said cup member within said enclosure and having the lower end portion thereof disposed within said cup-shaped member'with means providing a predetermined clearance therebetween whereby condensate formd on the cylindrical member References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 910,490 Bock Jan. 26, 1909 984,645 Bock Feb. 21, 1911 2,164,112 Jeremiassen June 27, 1939 2,204,180 Gerlach June 11, 1940 2,232,394 'rsu-Liang Ko Feb. 18, 1941 2,424,273 Hass July 22, 1947 2,452,576 Kjellgren Nov. 2, 1948 2,484,829 Holden Oct. 18, 1949 2,647,043 Imber July 28, 1953 OTHER REFERENCES Walker: "Piezoelectric Crystal Culture, Bell Laboratories Record, vol. XXV, No. 10, October 1947, page 360. 

1. APPARATUS FOR GROWING CRYSTALS COMPRISING A CLOSED CONTAINER ADAPTED TO BE PARTIALLY FILLED WITH A GROWING SOLUTION OF THE MATERIAL TO BE CRYSTALLIZED, MEANS FOR SUPPORTING A SEED CRYSTAL IN SAID GROWING SOLUTION, MEANS FOR CONTROLLING THE TEMPERATURE OF SAID GROWING SOLUTION, A CONDENSER MOUNTED ON SAID CONTAINER AND HAVING A CONDENSING UNIT WITH A COLLECTING CHAMBER POSITIONED IN THE VAPOR SPACE WITHIN SAID CONTAINER ABOVE THE LIQUID LEVEL OF SAID GROWING SOLUTION AND ABOVE THE UPPER ENCLOSING PORTION OF SAID CONTAINER, MEANS FOR REMOVING CONDENSATE FROM SAID COLLECTING CHAMBER, SOLENOID MEANS FOR OPERATING SAID LAST NAMED MEANS, MEANS FOR INTRODUCING MAKE UP SOLUTION TO SAID CONTAINER INCLUDING A VALVE ACTUATED BY SAID SOLENOID, CONTROL MEANS FOR ACTUATING SAID SOLENOIDS AT PREDETERMINED INTERVALS, SAID CONTROL MEANS INCLUD- 